ISDN

1
ISDN

• Today, customers deploy ISDN primarily as a WAN
  backup technology and to provide remote access to
  telecommuters and small offices.
• Although ISDN boasts a much faster call setup and
  higher throughput than POTS, many potential BRI
  customers are turning to digital subscriber line
  (DSL) and cable technologies. These typically
  offer much higher throughput at a lower cost.

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ISDN

• Despite these emerging technologies, ISDN remains
  a viable remote access solution for several
  reasons.
• ISDN is more widely available than DSL or cable.
• Many companies and service providers have made a
  significant investment in ISDN equipment and
  training, and plan to continue leveraging that
  investment.

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ISDN

• Remote offices using ISDN can connect to central
  offices directly, without traversing the public
  internet. Most DSL and cable implementations
  require the remote host communicate with the
  central site using a VPN over the internet.
• ISDN replaces traditional analog telephone
  equipment with high-speed digital equipment that
  provides the customer with a digital local loop.

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ISDN

• Customers can connect to the ISDN carrier via two
  different physical interfaces, BRI and PRI. A
  single BRI or PRI interface provides a
  multiplexed bundle of B and D channels.
• ISDN B channels are called bearer channels
  because they carry voice, data, and fax
  transmissions. B channels carry information in
  frame format, using either High Level Data Link
  Control (HDLC) or Point-to-Point Protocol (PPP)
  as the Layer 2 protocol.

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ISDN

• The D channel, or delta channel, is used for
  out-of-band signaling. The D channel carries
  control messages, such as call setup and
  teardown. Typically, the D channel employs Link
  Access Protocol (LAPD) at Layer 2.
• BRI service is provided over a local copper loop
  that traditionally carries analog phone service.
• BRI has the following characteristics

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ISDN

• Two 64-kbps bearer channels.
• One 16-kbps delta channel.
• 48 kbps of framing and synchronization
  information.
• Total speed of 192 kbps.
• ISDN BRI provides 128 kbps (two 64-kbps B
  channels) of available bandwidth.

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ISDN

• If you are discussing the bandwidth of both B
  channels and the D channel, ISDN BRI provides 144
  kbps.
• ISDN PRI service is provided over T1 and E1
  leased lines between the customer premise
  equipment (CPE) and the ISDN switch.
• A T1 is referenced as a DS1, which is made up of
  24 DS0 channels.
• A single BRI is one DS0.
• PRI over T1 specifies the following

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ISDN

• 23 64-kbps bearer channels.
• One 64-kbps D channel, carried in timeslot 24.
• 8 kbps of framing and synchronization
  information.
• Total speed of 1.544 Mbps.
• PRI over E1 provides the following

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ISDN

• 30 64-kbps bearer channels.
• One 64-kbps D channel, carried in timeslot 16.
• 64 kbps of framing and synchronization
  information.
• Total speed of 2.048 Mbps.

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11
BRI Call Processing

• When a BRI call is initiated, the CPE sends the
  called number to the local ISDN switch using the
  D channel.
• The local switch uses the Signaling System 7
  (SS7) protocols to set up a path inside the
  public switched telephone network (PSTN) and pass
  the called number to the terminating ISDN switch.
  This far-end switch brings up the D channel to
  the destination.

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BRI Call Processing

• Remember that the D channel is used for call
  setup, signaling, and call termination, which are
  the call-control functions.
• After the ISDN switch processes the call, SS7 is
  used to traverse the carrier's network.

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BRI Call Processing

• When the terminating CPE answers, the B channel
  is connected end to end.
• Both B channels can be used simultaneously to
  connect to the same destination or to different
  destinations.
• In addition to defining protocols, ISDN standards
  also specify the function of a given device in
  the network. BRI can involve many functional
  devices, also known as functional groups. The
  following functional groups are

14
ISDN Services

• Terminal equipment 1 (TE1) - designates a device
  that is compatible with the ISDN network.
  Examples of TE1 include a digital telephone, a
  router with an ISDN interface, or digital
  facsimile equipment.
• Terminal equipment 2 (TE2) - designates a device
  that is not compatible with ISDN and requires a
  terminal adapter, such as a router without an
  ISDN interface.

15
ISDN Services

• Terminal adapter (TA) - converts standard
  electrical signals into the form used by ISDN, so
  non-ISDN devices can connect to the ISDN network.
  An example is converting V.35 or EIA/TIA-232 to
  ISDN.

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ISDN Services

• Network termination type 1 (NT1) - connects
  four-wire ISDN subscriber wiring to the
  conventional two-wire local-loop facility. The
  NT1 is part of the CPE in the United States and
  part of the local exchange in Europe.

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ISDN Services

• Network termination type 2 (NT2) - directs
  traffic to and from different subscriber devices
  and the NT1. The NT2 is an intelligent device
  that performs switching and concentrating. Often,
  a private branch exchange (PBX) is the NT2
  device.
• Line termination (LT) - is located at the
  exchange side. Its functions are identical to
  those of an NT1.

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ISDN Services

• Exchange termination (ET) - are subscriber line
  cards in the ISDN exchange. LT and ET are
  sometimes referred to as the LE (local exchange).
  
• Local Exchange (LE) - is the ISDN central office
  (CO) that houses the ISDN switch. The LE
  implements the ISDN protocol and is part of the
  network.

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ISDN Services

• BRI requires an NT1 in order to connect to the
  carrier's ISDN switch. In Europe, the service
  provider maintains the NT1. In North America, the
  NT1 belongs to the customer, who is responsible
  for purchasing and installing it.
• Some devices, such as routers, can combine both
  TE1 and NT1 functionality in the same unit.

21
ISDN Services

• The ISDN reference points define how the
  functional groups, such as TE2 and TA, connect to
  each other. The ISDN reference points are

22
ISDN Services

• U reference point (user reference point) -
  located between the NT1 and LE (it corresponds
  with a subscriber line). There are no ITU-T
  standards for the U interface. This is the
  American National Standards Institute (ANSI)
  standard for the United States.

23
ISDN Services

• T reference point (terminal reference point) -
  located between the NT1 and NT2 (or between the
  NT1 and TE1 or TA, if there is no NT2 device). In
  BRI, the T interface is electrically identical to
  the S interface. Thus, the two reference points
  are typically combined in a single interface,
  referenced as an S/T interface.

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ISDN Services

• S reference point (system reference point) -
  located between the NT2 and TE1 or TA. It
  connects the terminals to the ISDN network. This
  is the most important interface for users. In
  BRI, the T interface is electrically identical to
  the S interface.

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ISDN Services

• R reference point (rate reference point) -
  located between TA and TE2 (non-ISDN interface).
  The TE2 connects to the TA via a standard
  physical-layer interface. These standards include
  EIA/TIA-232-C (formerly RS-232-C), V.24, X.21,
  and V.35.

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ISDN Services

• Some routers have one or more U interfaces, which
  means they can connect directly to the carrier's
  ISDN switch. In such a network, the S, T, and R
  reference points would not exist. Some
  manufacturers define a V reference point in LEs
  between LT and ET. This reference point
  identifies the network node interface and is
  transparent to users.

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29
PRI

• Recall that ISDN PRI is delivered via a leased T1
  or E1 line. A channel service unit/data service
  unit (CSU/DSU) is required to connect a router
  (the TE) to the carrier network. Internal
  CSU/DSUs are common among modular routers.

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PRI

• The PRI reference points are more straightforward
  than BRI reference points (see Figure ), which
  can contain numerous functional groups in a
  multipoint configuration.

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33
PRI

• If you need the throughput of a DS1 (1.544 Mbps),
  then a dedicated line is the appropriate
  solution. However, if you are a service provider
  or a large company that supports dozens of
  dial-up remote access connections, then ISDN PRI
  offers a powerful solution (see Figure ).

34
PRI

• The term, DS1, stands for digital service 1,
  which is an interface with a 1.544-Mbps data rate
  that often carries voice connections on a PBX.
  Each DS1 (also known as T1) has 24 DS0 channels
  framed together so that each DS0 timeslot can be
  assigned to a different type of trunk group, if
  desired.

35
PRI

• ISDN PRI supports hybrid dial-up access using a
  single phone number. One PRI over T1 can support
  up to 23 dial-in calls, which can be either
  analog POTS calls, or digital ISDN BRI calls. The
  callers dial the same phone number, although they
  are each connected to a different channel.

36
PRI

• Because each channel is a DS0 (64 kbps), PRI
  makes analog calls at 53.3 kbps (using 56K
  modems) possible. It is the D channel's job to
  identify if the call is a circuit-switched
  digital call or an analog modem call.

37
PRI

• In a typical configuration, an access server,
  such as a Cisco 3660, contains an integrated
  digital modem and a channelized T1 controller.
  The T1 controller connects to a leased T1 line,
  which provides connectivity to the ISDN cloud.
  This setup allows ISDN routers and analog modems
  to connect to the access server simultaneously.

38
ISDN Call Setup

• An ISDN call can be placed in numerous ways. In
  an ISDN call, the calling party requests a call
  setup.
• Before the actual connect and call proceeding,
  you might see several different messages, such as
  a progress message. This optional message will
  indicate how your call is proceeding.

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ISDN Call Setup

• The same is true for the alerting message, which
  is typical of telephone messages, but is not
  required. Alerting messages are not typical with
  data transmissions. Most of the time, you see
  connection messages with data calls.

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41
ISDN Call Teardown

• Similar to call setup, the call teardown request
  is not an end-to-end function, but instead is
  processed by the ISDN switch. The release
  procedures are based on a three-message approach
• disconnect
• release
• release complete

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Configuring ISDN BRI

• You must specify global and interface parameters
  to prepare the router for operation in an ISDN
  environment. This section focuses on configuring
  BRI for access routers.
• In global configuration mode, you must specify
  the ISDN service provider's switch type. There
  are approximately ten different switches to
  choose from, depending on the country where the
  connection is established.

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Configuring ISDN BRI

• The interface ISDN addressing tasks include
  assigning the IP address, dialer group (for DDR),
  and ISDN service profile statements (SPID
  numbers). You must also include a dialer map
  command that associates a statically mapped
  destination to a destination IP address,
  hostname, and ISDN dial number.

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Configuring ISDN BRI

• Different ISDN providers use different switch
  types. In fact, some providers use one type of
  switch hardware to emulate another type of switch
  in software. Your ISDN provider will provide you
  the appropriate switch type information.

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Configuring ISDN BRI

• If your router is not configured with the correct
  switch type information, it will not be able to
  communicate with the ISDN switch using Q.931 at
  Layer 3. In other words, no ISDN call can be
  dialed or received.
• You set the switch type by entering the isdn
  switch-type command. However, you must decide
  whether to enter this command in either global or
  interface configuration mode.

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Configuring ISDN BRI

• The global isdn switch-type command sets all ISDN
  interfaces on the router to be configured for the
  same switch type
• Router(config)isdn switch-type type.
• When you issue the isdn switch-type command in
  interface configuration mode, only the interface
  that you are configuring assumes the switch type
• Router(config-if)isdn switch-type type

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Configuring ISDN BRI

• Note that the interface configuration command
  overrides the global-level command.
• Typically, you specify the switch type in global
  configuration mode using the isdn switch-type
  command. However, in some cases you might need to
  specify two different ISDN switch types.

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Configuring ISDN BRI

• This would be necessary when the same router
  connects to both BRI and PRI. For this reason,
  Cisco extended the use of the command to the
  interface configuration mode.

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Configuring ISDN BRI

• The interface bri interface-number command
  designates an ISDN interface on a router that
  natively supports ISDN (TE1). Thus, to configure
  the first ISDN interface with the ATT basic rate
  switch type, you would enter the following
  commands
• RTA(config)interface bri 0RTA(config-if)isdn
  switch-type basic-5ess

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Configuring ISDN BRI

• If the router does not have a native BRI (if it
  is a TE2 device), it must use an external ISDN
  terminal adapter. On a TE2 router, use the
  interface serial interface-number  command.

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